feat: implement websocket.Accept in js (#373)

[shynome]

#373 (comment)

DialOptions.HTTPClient also supported

[mafredri]

@shynome thanks for the PR. There's quite a lot going on here, could you explain in broad terms what changes you've made, and why. And what new use-cases do these changes enable?

I have some reservations about making such a large API change and introducing new exported types.

[shynome]

yes, I'm buillding a vpn based on webrtc, it can be running in browser.

the srouce branch: https://github.com/shynome/websocket/commits/feature/js-accept/

I have do the below things

1. rename Conn to StdConn for split BrowserConn and GoConn
2. Add the type Conn interface, but change type Conn sturct to type Conn interface is make api compatible broken,
   so I rename type Conn interface to type Stream interface (maybe other name is better clearly, could you provide one?), and use the Conn struct wrap type Stream interface
3. now we can export websocket.Accept in js, because now we have StdConn in js

[shynome]

I know this PR has a lot of changes, so it might not be accepted, but in reality, the changes I made are quite few.

First, add the type Conn(Stream) interface to unify StdConn and BrowserConn. Then, export StdConn and BrowserConn so that users know which actual Conn they are working with.

Finally, thanks for your review!